The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the correction of geometric distortion in phase and phase difference MRI images due to gradient field non-linearities.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.o), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.l) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins, and after the excitation signal B.sub.l is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x, G.sub.y, and G.sub.z) are employed to locate the position of the excited spins. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The reconstructed MRI image can be geometrically distorted due to a number of causes. The location of an object, or portions of the object, can be shifted due to changes in the local Larmor resonance frequency. Chemical shift artifacts caused by small differences in Larmor resonance frequency (e.g. fat and water spins), susceptibility differences between different tissues, and patient induced or polarizing magnetic field inhomogeneities are examples of sources for such shifting of position. Another source of distortion is stretching or contracting of the imaged object caused by non-linearity of one or more of the applied gradient magnetic fields. This distortion is referred to as "warping". Both distortions can be very substantial, however, in standard cylindrical magnets used for MR imaging and spectroscopy, warping distortion is usually much greater than the distortion due to shifting. This is particularly true when local gradient coils are employed in lieu of whole body gradient coils, since the field of view over which local coil gradient fields are linear is very limited. In addition to these "static" geometric distortions, time variant and spatially variant gradient eddy currents also contribute to both object shifting and image warping.
Techniques such as those disclosed in U.S. Pat. Nos. 4,591,789 and 4,672,320 have been developed to correct magnitude MRI images for these various types of geometric distortions. These correction methods work well when the data being corrected is continuous in nature, or does not need to be combined subsequently with other geometrically corrected image data. These correction methods thus work well for conventional magnitude images, but do not work when applied to image data based on the phase of the acquired NMR signals or the difference in phase between acquired NMR signals. The geometric correction methods are not applicable, therefore, to phase contrast MR angiography ("MRA") as described in U.S. Pat. Nos. 5,093,620; 5,101,156; 5,226,418; and 3-point Dixon acquisitions as disclosed in U.S. Pat. Nos. 5,144,235; 5,225,781 and 5,321,359; and they are not applicable to a number of other NMR methods relying on accurate phase information as described in U.S. Pat. Nos. 5,168,232; and 4,987,371.